Stretched films produced by stretching resin are, for their optical anisotropy, used as optical films that perform various optical functions in a variety of display devices. For example, in liquid crystal display devices, a stretched film can be used as an optical compensation film for optical compensation such as coloring prevention, viewing angle enhancement, etc., or a stretched film can be bonded to a polarizer so that the stretched film is used as a phase-difference film that serves also as a polarizer protection film, to mention a few known designs.
On the other hand, in recent years, as new types of display devices, self-luminous display devices such as organic EL (electroluminescence) image display devices have been attracting much attention. Self-luminous display devices have more margin for suppression of electric power consumption than liquid crystal display devices, which require backlight to be constantly on. Moreover, with self-luminous display devices such as organic EL image display devices where light sources corresponding to different colors are lit respectively, there is no need to provide color filters, which may lead to lower contrast, and thus it is possible to obtain higher contrast.
However, in an organic EL image display device, to enhance light extraction efficiency, a reflector such as a plate of aluminum is provided on the rear side of the display. Thus, when external light that has entered the display is reflected on the reflector, the image may disadvantageously have lower contrast.
To prevent reflection of external light and thereby to enhance bright/dim contrast, a stretched film is bonded to a polarizer to form a circular polarizing plate, and this circular polarizing plate is used on the front side of the display, according to a known design. Here, the circular polarizing plate is formed by bonding together the polarizer and the stretched film such that the in-plane slow axis of the stretched film is inclined at a desired angle relative to the transmission axis of the polarizer.
However, a common polarizer (polarizing film) is obtained through high-factor stretching in the transport direction, and its transmission axis is aligned with the width direction. Moreover, a conventional phase-difference film is produced by longitudinal stretching or lateral stretching and, in principle, the in-plane slow axis points in a direction at 0° or 90° relative to the length direction of the film. Thus, to obtain a desired inclination angle between the transmission axis of the polarizer and the slow axis of the stretched film, there is no choice but to adopt a batch method involving cutting a long polarizing film and/or a stretched film into pieces at a particular angle and then bonding together such pieces one by one. This disadvantageously results in poor productivity and low product yields due to contamination with shavings.
As a solution, there have been proposed various methods for production of a long phase-difference film that permit a film to be stretched in a direction at a desired angle relative to (in a direction oblique to) the length direction and that thus permit the direction of the slow axis to be controlled to be an arbitrary direction neither at 0° or 90° relative to the length direction of the film. For example, according to the production method disclosed in Patent Document 1, a resin film is dispensed from a direction different from the winding direction of the film after stretching, and is transported with both end portions of the resin film held with a pair of holding members. The transport direction of the resin film is changed on the way and thereby the resin film is stretched in an oblique direction. In this way, a long stretched film is produced that has a slow axis at a desired angle more than 0° but less than 90° relative to the length direction.
By use of such a stretched film having a slow axis inclined relative to the length direction, it is possible to produce a circular polarizing plate by bonding together a long polarizing film and a stretched film on a roll to roll basis instead of bonding together by the conventional batch method. This dramatically enhances the productivity of the circular polarizing plate, and greatly improves its yield.
Incidentally, in a stretching apparatus for obliquely stretching a film (a stretched film production apparatus), the film is passed through, in the order named, a preheating zone, a stretching zone, and a heat-fixing zone where temperature control is performed individually, and the film is stretched in the stretching zone. Here, when the film transport speed is increased with a view to producing a stretched film at high speed (for example, when the film is transported at 30 m/min.), the air in an upstream-side zone is drawn into a downstream-side zone as the film is transported, causing unevenness in temperature inside those zones (in particular, in the vicinity of the film), and this unevenness in temperature causes variations in the optical values (orientation angle and in-plane retardation Ro) of the film. Thus, when the obliquely stretched film is applied to a circular polarizing plate for external light reflection prevention in an image display device with very high contrast, such as a large-screen organic EL television (OLED-TV), reflected external light leaks through the circular polarizing plate to different degrees from place to place over the display screen during display of black, that is, so-called unevenness in the amount of reflected light occurs.
In this connection, according to Patent Document 1, the angle between the width direction of a partition wall partitioning between zones neighboring in the film transport direction and the film transport direction is made variable, and in addition the interval of the gap in the partition wall (the gap through which the film passes) in the up/down direction of the film is made adjustable within a predetermined range. Thus, according to Patent Document 1, even when the film is obliquely stretched such that the orientation axis (slow axis) is inclined at a certain angle (for example, 15° or 45°) relative to the width direction, the orientation axis can be oriented with high accuracy. However, Patent Document 1 makes no mention of heating in the vicinity of the film. Thus, when the film transport speed is increased, unevenness in temperature in the vicinity of the film cannot be suppressed, and thus variations in optical values cannot be suppressed.
The example presented in Patent Document 1 assumes that the film transport speed equals 10 m/min., and also in this respect, the production method of Patent Document 1 is not considered to be ready for increased film transport speeds.
Thus, to suppress unevenness in the amount of reflected light during display of black on an organic EL television, it is necessary, during high-transport-speed production of a film for application to a circular polarizing plate in such an organic EL television, to suppress unevenness in temperature in the vicinity of the film and thereby suppress variations in optical values, for each film having a different orientation direction.